My dad and I used to explore a lot of stuff around town when I was a kid, back in the mid 90's. There were so many empty/abandoned places that are since gone or converted to something else. The old MFA mill on Boonville was very spooky, you'd hear doors slam randomly from the wind. The "Albino Farm" (Springlawn) was awesome in the 90's, there was an old log cabin on the property, and a few other buildings that are now gone.

Old factories, grain mills, a metal recycling plant, the tunnels you mentioned, and Winoka, were all places we went to "back in the day"

Places I've gone on my own or with friends: the Woodruff Building/Sky 11 before remodel, the "Messiah Mill" on Chestnut and National, various caves, the Springfield Underground, old farmhouses further outside of town, and so many more.

I don't go as much now, as this day and age you need friends along, running into tweakers and scrappers can go sideways fast, and most of the people I went with have moved out of the area, and my dad doesn't feel up to it much any more.

I never did get to Dogpatch, which is a bummer, but I did see most of the pics online from there, and I always wanted to get into Celebration City, but heard security was pretty tight.

I never thought other people would enjoy going traipsing through old ruined buildings like Dad and I did, until the early 2000's when I started finding sites like UO/UNF, UER and others, and learned that Urbex was a hobby shared by many others.